The revolution has begun! Web video will be freed from the shackles of the MPEG-LA and the dreaded claws of patents and incomprehensible licenses! Sorry, I got a little carried away there. Anywho, YouTube has announced all new videos uploaded to the site will be transcoded into WebM, and that the most important part of the site's catalogue is already available in WebM.

people will notice when they can no longer play the vids. This is particularly true on 'alternative' OS where the browsers might not be updated to support the new format.

Ok, I know its a small minority, but its annoying none the less.

When YouTube first began, people would visit the site for the first time and many would see no videos. The site would explain that they needed a plug-in for their browser, and it gave a link to Adobe's Flash Player.

When YouTube moves to WebM, people using Firefox 4, Opera, Chrome or Chromium (on any OS) will have no such a problem. People who are using IE9 or Safari ... would see a message that they needed a codec for their system, and they would be given a link to Google's codec download site.

I would like to know what you classify as 'alternate'. Linux, BSD and most Unix variations can get the up-to-date browsers easily. Those of you using Haiku, i think there are updated browsers available, but not being a haiku user i am not quite sure. Any other 'alternate' OS's will either have a compiled version OR you OS is soo old that you really should upgrade or atleast have one under a virtual machine.

People on such alternative platforms almost certainly don't have flash and might not have an h.264 decoder, and even if they do its probably not correctly licensed...
At least with WebM, it becomes relatively easy to bring support for it to these alternative platforms, and you can bet that such support will be rapidly ported to any platform still seeing active development.